The field of the disclosure relates generally to downhole tools associated with wireline, Logging While Drilling (LWD), and well integrity and production logging operations, and in particular, to gamma ray downhole detectors.
Radiation detectors are well known in the drilling industry and are often incorporated into drilling tools for oil wells and into the tools used to log the geologic formations along the length of the borehole. At least some known radiation detectors typically include a light detecting and quantifying device, such as a photo-multiplier tube, and a scintillation element, such as a crystal or suitably compounded element. The scintillation element typically functions by capturing radiation from the formation and converting that energy into light. The radiation may be ambient radiation emitted by radioactive materials in the formation, or radiation emitted in response to bombardment of the formation by radiation sources within the tool or equipment in which the detectors are operating.
Light generated within a scintillation element, as a result of intercepting radiation, is transmitted through an optical window into the photo-multiplier tube. The light impulses are transformed into electrical impulses that are transmitted via a data stream to an instrumentation system. Optical coupling elements are typically used between the scintillation element and the light-detecting element in order to facilitate increasing the light transmission, and may be used to provide isolation between the scintillation element and the light-detecting element.
Measurement while drilling (MWD) operations or logging while drilling (LWD) operations utilize radiation detectors to help guide the drills and/or to help evaluate the formation, concurrent with the drilling operation, thereby subjecting the radiation detector to increased vibration and shock, while at temperatures up to 175 degrees Celsius, or higher. Other drilling applications that subject the radiation detectors to extreme environments include environmental evaluations, geologic surveys, and construction projects. In the above-noted instances, a highly ruggedized radiation detector is desired so that the radiation detector will not fail and will not produce noise as a result of the vibration and shock.
With some known radiation detectors, environmental effects during drilling, such as the presence of fluids, solids, or gas, prevent contact between the radiation detector and the surface to be measured (e.g. cuttings, mud, hydrocarbons, etc.). This can cause insufficient and/or inaccurate data to be received by the radiation detector. Moreover, due to the vibration and shock described above, the elements of typical radiation detectors, such as a scintillation crystal or a photo-multiplier tube, can be damaged or produce increased noise due to vibrations. Furthermore, the increased temperatures of the drilling environment may decrease the useful life of many known radiation detectors.